The purposes of the formation of a chromate conversion coating on a metal surface are to provide corrosion resistance, and improve adhesion of coatings. The conversion coating improves the adhesion of coating layers such as paints, inks, lacquers and plastic coatings. A chromate conversion coating is typically provided by contacting a metal surface with an aqueous composition containing hexavalent or trivalent chromium ions, phosphate ions and fluoride ions. Concerns exist regarding the pollution effects of the chromate or phosphate discharged into rivers or waterways by such processes. Because of the high solubility and the strongly oxidizing character of hexavalent chromium ions, conventional chromate conversion processes require extensive waste treatment procedures to control their discharge. In addition, the disposal of the solid sludge from such waste treatment procedures is a significant problem.
Attempts have been made to produce an acceptable chromate free conversion coating for metals such as aluminum. Chromate free conversion coatings based upon complex fluoroacids and heavy metals are known in the art, however, they have not enjoyed wide-spread commercial acceptance. U.S. Pat. No. 4,191,596 which issued to Dollman et al., discloses a composition for coating aluminum which comprises a polyacrylic acid and H.sub.2 ZrF.sub.6, H.sub.2 TiF.sub.6 or H.sub.2 SiF.sub.6. The '596 disclosure is limited to a water soluble polyacrylic acid or water dispersible emulsions of polyacrylic acid esters in combination with the described metal acid at a pH of less than about 3.5.
U.S. Pat. No. 4,921,552 which issued to Sander et al., discloses a non-chromate coating for aluminum which is dried in place which forms a coating having a weight from about 6 to 25 milligrams per square foot. The aqueous coating composition consists essentially of more than 8 grams per liter dihydrohexafluozirconic acid, more than 10 grams per liter of water soluble acrylic acid and homopolymers thereof and more than 0.17 grams per liter hydrofluoric acid. The disclosure notes that it was believed that copolymers of acrylic acid would also be effective, however, no examples were given.
U.S. Pat. No. 4,136,073 which issued to Muro et al., discloses a composition and process for the pretreatment of aluminum surfaces using an aqueous acidic bath containing a stable organic film forming polymer and a soluble titanium compound. The disclosed polymers include vinyl polymers and copolymers derived from monomers such as vinyl acetate, vinylidene chloride, vinyl chloride; acrylic polymers derived from monomers such as acrylic acid, methacrylic acid, acrylic esters, methacrylic esters and the like; aminoalkyl, epoxy, urethane polyester, styrene and olefin polymers and copolymers; and natural synthetic rubbers.
An aqueous nonchromated coating for nonferrous metals such as aluminum is disclosed in U.S. Pat. No. 5,122,202 which issued to Dykstra et al. The coating contains an anionic polyacrylic/polyacrylamide copolymer, ammonium zirconium fluoride, nitric acid, water and optionally a mold inhibitor.
The environmental concerns which exist with respect to chromates have begun to arise with respect to heavy metals also. Typically, metal coatings which are chromate free employ some form of heavy metals such as zirconium or titanium as well as some form of fluoride.